The ITU has defined in ITU-T G.709 the signals format and interfaces of the Optical Transport Network (OTN). The basic frame structure is an Optical Transport Module of size k (OTUk), where k can be 1, 2, 2e, 3, 3e2, or 4. It contains framing and section overhead plus a bit-synchronously mapped transport entity termed Optical Data Unit of size k (ODUk). An ODUk contains a payload area plus ODUk overhead. An Optical Payload Unit (OPUk) is mapped into the payload area and carries a client signal or other lower order ODUs being time-division multiplexed. OTUk signals are asynchronous within certain specified limits of typically ±20 ppm.
To create an OTU frame, a client signal rate is first adapted at the OPU layer. The adaptation contains adjusting the client signal rate to the OPU rate. The OPU overhead contains information to support the adaptation of the client signal. The adapted OPU is then mapped into the ODU. The ODU overhead contains overhead bytes that allow end-to-end supervision and tandem connection monitoring. Finally, the ODU is mapped into an OTU, which provides framing as well as section monitoring and forward error correction (FEC).
In an Optical Transport Network, connections are switched on ODU level. The ODU is thus the switchable transport entity that travels along a network path. A characteristic feature of OTN is the asynchronous operation and the bit synchronous mapping of ODUk into OTUk, which results in the fact that a received ODUk, which gets connected to another output of the network node, determines the clock of the OTUk signal at the output.
The start of an OTUk frame is detected by scanning for a frame alignment signal (FAS). Once the FAS is found the receiving network element is aligned to the clock and frame phase of the incoming signal and stops scanning through the incoming signal. In this state an network element checks whether the FAS is at the expected position of an incoming frame. A network element that forwards a frame overwrites the FAS of the received signal on egress to compensate potential bit errors in the incoming FAS.
During network operation, it can happen that phase shifts occur e.g. due to connection switching, insertion or removal of maintenance signals, etc. These phase shifts are detected by an network element as it does not find the FAS at the expected location in the incoming signal. When a FAS is not found at the expected position for six consecutive frames, the network element starts scanning for a new FAS position and aligns itself to the new frame position within two frames. It thus takes 8 frames until a network element aligns itself to a phase shift of an incoming signal. The OTN standards require that during this 8 frame periods, the network element continues to write a FAS into the outgoing signal at the expected frame start position.